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Why does a bigger magnet have a lower Surface Field than a smaller one? That doesn't make any sense!

We're often asked about why a particular magnet has a certain Surface Field value. What's really counterintuitive is how some of our biggest, heavy lifting magnets often show some relatively low values for their Surface Field. How can a monster 3" diameter disk have a lower field strength than a little refrigerator magnet?

What is the Magnetic Field?

When specified on our site, the surface field or magnetic field refers to the strength of the B-field in Gauss. It is a measure of the magnetic field's effects on its environment. For axially magnetized discs and cylinders, we specify it on the surface of the magnet, along the center axis of magnetization. For blocks, it is specified on the surface of the magnet, also along the center axis of magnetization.

How is it measured?

For some shapes, the magnetic field strength along the central axis of the field can be calculated using known formulas. It is a function of the shape of the magnet and the grade of magnetic material. Other methods include the use of FEA software, experimental measurement with a magnetometer, or a combination of both.

Our specified values come from these theoretical formulas, backed up by many years of empirical testing of our magnets.

Discs & Cylinders

Blocks

Rings

Spheres

OK, but why does my big powerful magnet have a lower field than a smaller, weaker magnet?

The magnitude of the magnetic field (B) is more a function of the shape of the magnet than its size. For example, a 1/4" cube (B444) has a Surface Field of 5754 Gauss. So does a bigger cube, such as a 1/2" cube (B888), 1" cube (BX0X0X0), or even a monster 2" cube (BY0Y0Y0).

Magnets that are thinner in the direction of their magnetization will generally have lower Surface Field values. Thicker magnets will have higher values.

For example, consider two magnets both having a diameter of 1/4". A D401 (1/4" diameter, 1/32" thick) has a Surface Field of 1601 Gauss. A thicker D48 (1/4" diameter, 1/2" thick) has a Surface Field of 6403 Gauss!

If the size-to-thickness ratio is the same, you'll see the same Surface Field values. A D81 (1/2" diameter, 1/16" thick) has the same diameter-to-thickness ratio as the D401 and the same Surface Field! So does the:

But won't a bigger magnet project a bigger field?

Yep, it sure will. The dimensionless shape of the magnetic field is the same for two same-shaped magnets. The actual size varies with the magnet size.

Let's go back to the cubes for an example. A 1/4" cube (B444) has a Surface Field of 5754 Gauss. Using the Magnet Calculator, we can find the field at a specified distance. For Distance, enter a value equal to the length of one of our sides, in this case: 0.25". The Magnetic Field strength at that distance is 599 Gauss.

Now, look at a big 1" cube, (BX0X0X0). We'll plug in a distance value equal to 1" in this case, and the calculator again indicates 599 Gauss. The bigger magnet is projecting the magnetic field over a much larger area and distance than the little one.

What doesn't the Surface Field number tell me?

Surface Field is just the Magnetic Field strength at one discrete point, trying to describe a complex 3-dimensional field around the magnet. There are lots of interesting things that occur, even before you start considering what happens to the field in your application. Other magnets and/or ferrous metals around near the magnet can cause big changes in the size and shape of the magnetic field. You can find a few examples depicted here.

One example is referred to as edge-effects. Especially for neodymium magnets that are thinner than they are wide, you'll find much higher magnetic field strengths right at the edges. Consider the magnetic field of a D82 magnet. The strength at the center is 2952 Gauss, but it gets much higher right at the edges.

Magnetic Field strength, measured along the surface of a D82 magnet, from the center out to the edge.

What are those values out to the edges? We can't calculate it using our calculator or any simple formula. Values are typically found either using FEA software or experimental measurement. Data for our D82 magnet is shown at right, with distance measured from the center out to the edge.

Here are some tools that might help with further investigation of magnetic fields:

Viewing Film - A green film used to view magnetic fields of permanent magnets. The material turns dark when magnetic field lines are running perpendicular to its surface, and turns light when the magnetic field lines are running parallel to the surface.

Iron Filings - the classic magnet visualization technique. While we don't sell iron shavings, you can certainly find/make some. Grind any piece of iron or steel with a file, and you'll make dust. This will stick to magnets like crazy. A less messy way to visualize fields is to place the dust on a surface, and put the magnet beneath it.

Definitions:

Magnetic Field - The strength of the B-field in Gauss. It is a measure of the magnetic field's effects on its environment.

Surface Field - The Magnetic Field as measured or calculated on the surface of a permanent magnet, typically on the center axis of magnetization.